mlxsw: spectrum_buffers: Add extack messages for invalid configurations

Add extack messages to better communicate invalid configuration to the
user.

Example:

# devlink sb pool set pci/0000:01:00.0 pool 0 size 104857600 thtype dynamic
Error: mlxsw_spectrum: Exceeded shared buffer size.
devlink answers: Invalid argument

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>